Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving some overhang for easy removal later.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt.

In a separate large bowl, combine the melted butter and granulated sugar. Stir until smooth, then beat in the eggs and vanilla until fully incorporated.

Add the dry ingredients into the wet mixture and mix until just combined. Fold in the chocolate chips.

In another small bowl, beat the softened cream cheese with powdered sugar and lemon zest (if using) until smooth and creamy.

Spread about two-thirds of the brownie batter into the prepared baking pan to form the base. Gently spread the cheesecake mixture over the brownie base.

Spoon the remaining brownie batter in dollops over the cheesecake layer. Use a toothpick or knife to gently swirl the batters together to create a marbled effect.

Bake in the preheated oven for 20 to 25 minutes, until the edges are set and the center is slightly soft. Do not overbake to keep the bites moist and fudgy.

Allow to cool completely in the pan on a wire rack. Once cooled, lift the dessert out using the parchment overhang and cut into bite-sized squares.